Terephthalic acid (TPA) is an aromatic carboxylic acid widely used as a chemical intermediate. Terephthalic acid is of commercial interest to the polymer industry because of its use in the manufacture of saturated polyesters, such as polyethylene terephthalate (PET), and its copolymers. Worldwide production of TPA and its corresponding dimethyl ester (dimethyl terephthalate) ranked about 25th in tonnage of all chemicals produced in 1992, and about 10th among all organic chemicals.
As shown in the scheme below, the oxidation of p-xylene by molecular oxygen is a radical initiated, step-wise reaction which produces two main intermediates, p-toluic acid and 4-formyl-benzoic acid (also known as 4-carboxybenzaldehyde or 4-CBA).

There are numerous process methods available for manufacturing TPA, each of which have varying production and purity yields for TPA. Most of these processes involve oxidation of p-xylene with an oxygen source, e.g., air or O2 gas, in the presence of liquid phase homogeneous catalysts containing at least cobalt and/or manganese metals. In addition, most of these processes are conducted in the presence of an acidic solvent, such as acetic acid. The acetic acid tends to oxidize, leading to solvent loss, and must be separated from water at the end of the process; recovery of acetic acid is therefore very expensive. Conventional processes also employ corrosive bromine promoters as a radical source, e.g. HBr, NaBr, or other metal bromines. Consequently, these processes are typically conducted in expensive, titanium-clad reactors. As it is formed, the TPA precipitates from the acetic acid reaction medium, resulting in a concentrated slurry of TPA particles.
The TPA precipitate is typically contaminated with 4-CBA due to incomplete oxidation during the reaction. Contamination with 4-CBA can be substantial; for instance, some production processes yield a TPA stock which has approximately 5000 ppm of 4-CBA (Pernicona et al., Catalysis Today, Vol. 44: p. 129-135 (1998)). The presence of even minor amounts of 4-CBA in the terephthalic acid interferes with high-grade polyester synthesis such as the polycondensation of TPA to PET. Hence, the 4-CBA must be removed from the TPA. However, removal of 4-CBA is complicated by the fact that it readily co-crystallizes with TPA. Typically, the TPA is purified by high-pressure hydrogenation of the aldehyde to the easily separable p-toluic acid as described in U.S. Pat. No. 5,200,557. However, this additional step and the subsequent recrystallization that accompanies it are expensive.
Currently, there exists a need for methods of synthesizing terephthalic acid with sufficiently high yields and suitable purity for subsequent high-grade manufacturing processes, so as to avoid the use of additional purification steps. In addition, there exists a need for methods that avoid the use of corrosive feed materials or other process materials which may be harmful to the environment, such as NaBr or HBr.
It has been described in WO-A 98/38150 to produce terephthalic acid in such a way that substantially all of the aromatic carboxylic acid produced in the course of the reaction is maintained in solution during the reaction. No specific measures have been described for maintaining the acid in solution.